update links in Tails sysadmin doc

- Consider deleting backups

  If you decide to delete backups, see "Deleting backups of a decommissioned

  system" in `sysadmin.git:backups.mdwn`.

  system" in [[tails/services/backups]].

- Delete the VM definition in libvirt.

* If this system needs some trustworthy connection to lizard or one of

  our other system (which is the case e.g. when the new system is

  meant to use lizard's puppetmaster), follow the VPN documentation:

  [[VPN]].

  [[tails/services/vpn]].

* Follow the [[lizard/install_base_vm]] starting from point 8.

* Follow [[tails/how-tos/install-a-vm]] starting from point 8.

  - Skip what is related to VM management.

  - When declaring the node in our manifest, the ecours.tails.net

    one can be a good source of inspiration, e.g for the

  not** use lizard's puppetmaster.

* Set up monitoring. Follow the [[monitoring installation

  notes|tails/how-tos/install_monitoring]], paying attention that:

  notes|tails/how-tos/install-monitoring]], paying attention that:

  - Traffic on the VPN between the new host Icinga2 agent and ecours

    Icinga2 master (port 5665) must be whitelisted in their respective

    firewalls.

  manifest on stone and then ssh from your new machine to stone to

  verify the fingerprint.

  After this, follow the instructions in [[sysadmin/backups.mdwn]]

  After this, follow the instructions in [[tails/services/backups]]

  concerning new backups.